A mobile cellular telephone includes a transmitter developing an RF output signal applied to an antenna. It is desirable to control the power output to the minimum level necessary to maintain reliable communications. This minimizes interference among users and maximizes battery lifetime.
Present cellular telephone systems sample the transmitted RF signal and compare it to a DC control voltage known to correspond to a specific transmitter power level. A difference signal is fed back to a gain control transmitter stage to adjust the power up or down as required to achieve the desired power output. Correlation between the DC control voltage and the RF output power is established in a calibration procedure performed as part of the manufacturing test process. The phone is set to transmit known power levels, generally low, medium, and high power, and the detector voltage is read and stored into non-volatile memory as a digital word. This procedure may be repeated at several frequencies across the operating band. Values in between calibration points may be interpolated by software to produce intermediate power outputs.
A typical detector circuit uses a diode for rectifying the RF signal to a DC signal proportional to the power output. To improve sensitivity of the diode detector, a DC bias current is often passed through the diode. This overcomes the threshold voltage of the diode, improving detection at low signal levels. However, due to the temperature dependence of the diode's PN junction, the output voltage from the detector varies over temperature. As the detected power level decreases, the measurement becomes less accurate because the temperature dependent offset voltage becomes a larger portion of the detector's DC output voltage. To overcome the temperature dependence of the detector diode, schemes have been proposed using an identical diode to cancel the temperature varying voltage. However, exact matching is difficult, and as the detected power decreases, the inaccuracy increases.
The present invention is directed to overcoming the above problems in a novel and simple manner.